Dagannoth, known as Daggermouths to the Fremennik,are sea monsters. Many are found near areas inhabited by the Fremennik, who hunt them for their skin, which is used to make armour. There are many types of Dagannoth, the most powerful being the three Dagannoth Kings.

Little is known about the origins of the dagannoth species. Dagannoth are first documented in Third Age where they started attacking the Fremennik, causing great losses to them. The Fremennik people battle them to this day.

Through the Spirit Dagannoth they reveal their goal: To force the entire surface world to be plunged into the Deep through a terrible flood, the event is called "The Day of Rising" by the Dagannoths. The lucky ones will be consumed first by the Dagannoth while the others will drown before they are also consumed.

In the FunOrb game Armies of Gielinor, one may summon a Dagannoth, which uses the appearance of a Spirit dagannoth.

Dagannoth may be based on the Deep Ones from H.P. Lovecraft's The Shadow Over Innsmouth. In the aforementioned story, they worshipped the elder beings Father Dagon and Mother Hydra.

Dagannoth may also be based on the Xenomorph Aliens from the Alien movie series, as several references are made to them throughout the game.

While the idea for the creatures characteristics may derive from the above mentioned, the name itself seems as if it may come from one of the first computer role-playing games, Dungeons of Daggorath, produced by DynaMicro in 1982.

They may be dinosaurs; while fighting the Dagannoth Mother from Blood Runs Deep, a music track called "Maiasaura" unlocks, which is the name of a genus of dinosaur known especially for their maternal nature.